respecting Donhle Refraction. S95 
the moveable image Q ; the object A, the moveable image Z, 
and the eye O, being in the same line; and that the same ob- 
ject A did transmit to the eye O, remaining in the same posi- 
tion, yet another species Y, through the refracted ray AYO. 
Whence it was manifest to me, that this unusual refraction had 
for its rule the parallel of the sides of this double refracting 
ci’ystal, while the usual refraction was directed according to the 
perpendicular of the superficies. 
‘‘But considering that the place of the point appearing through 
our diaphanous body cannot easily be determined, as being on- 
ly obvious in the uppermost part, we shall add the way where- 
by we have found its diversity, by drawing, on the subjacent 
Table, a straight line through that point ; the place of which 
line will be determined by the one eye through this crystal, and 
by the other eye without the crystal. For, in the same Figure, 
let through the object A be drawn upon the Table a straight line 
BC. The eye being in M, that double line HD and IE will 
appear, the species being cast on the upper surface ; and, if 
you will attend well, you will observe one of the images, viz. 
the fixed HD to be congruent to the adjacent line BC, whilst 
the other, namely the moveable El, tendeth towards 11. But 
if afterwards the eye be posited in O, the same object, I mean 
the line BC, will not only be represented double by the images 
KF and LG, but also the moveable image GL be congruent to 
the inferior line BC ; while the fixed FK is not so, but tends 
towards N.” 
After describing these experiments, Bartholinus proceeds to 
measure the ratio of the angles of incidence and refraction in 
the ordinary image, and he finds it to be as five to three, which 
makes the index of refraction 1.667. He endeavours to ac- 
count for the double refraction, by supposing that the Iceland 
crystal has two sets of pores ; one, “ according to the ductus or 
direction of the sides, and parallel thereto; since, it may be observ- 
ed, that according to this disposition of the sides it is broken, and 
the parts severed from one another ; and that one of the images, 
namely the moveable^ passeth through them. Next, besides 
these pores lying according to the parallelism of the sides, it 
hath others, such as glass, water, and right crystals have^ 
through which the right image is transmitted.” 
